The present invention relates to a current-limiting device for suppressing an overcurrent in an electric circuit such as, for example, an electric power control system in which an instantaneous overcurrent tends to flow in the event of an abnormality.
The standard resistor is generally made of a resistance material such as, for example, metal, porcelain enamel or carbon that exhibits a predetermined resistance. A fuse effective to open a circuit in the event of an overcurrent instantaneously flowing in such circuit is generally used in order to protect such circuit from the overcurrent.
However, when an overcurrent is generated and the circuit opens as a result of breakage of the fuse, complicated procedures are needed to reset the circuit in the original, operating condition. Although an easily resettable breaker is often inserted in the circuit, it is incapable of responding to an overcurrent of a duration shorter than a few milliseconds. Also, the standard resistor has a constant resistance for the applied voltage and, therefore, the resistor alone cannot control the overcurrent and this is particularly true of an electric power control system in which a high voltage and a high current flow. Therefore, the resistor has to be used in combination with a fuse in such electric power control system. Moreover, the resistor generally has a resistance that changes when heated by the flow of a relatively large current therethrough, and accordingly, requires cooling in order for the resistor to exhibit a predetermined resistance characteristic. This cooling is, however, difficult to accomplish with most standard resistors.
The temperature of the resistor abruptly increases when a relatively large current even though instantaneous flows therethrough. In order to suppress the overcurrent without the circuit being opened, however, it is considered necessary to suppress the overcurrent even when a high temperature is evolved as a result of increase of the temperature of the resistor. In the electric power control system, the resistor employed therein is assumed to be heated to about 120xc2x0 C. in consideration of the magnitude of current which would be employed in the electric power control system, and hence, the resistor has to be guaranteed having a maximum operating temperature up to about 200xc2x0 C. On the other hand, it is generally assumed that the resistor would be heated to about 230xc2x0 C. (500K) in the event of an abnormality in which the overcurrent flows therethrough.
It is to be noted that the use of a semiconductor element as a resistor in a circuit is well known in the art. For example, the Japanese Laid-open Patent Publications No. 10-70289 and No. 6-21349 disclose a non-symmetry snubber resistor and an amorphous semiconductor device, respectively.
Specifically, the Japanese Laid-open Patent Publications No. 10-70289 discloses a non-symmetric snubber resistor that comprises snubber resistor areas and snubber diode areas. The snubber resistor areas have a directionality because of its non-symmetry and is capable of discharging the charge built in the snubber condenser.
On the other hand, the Japanese Laid-open Patent Publications No. 6-21349 discloses an amorphous semiconductor that comprises a chalcogen compound that is sandwiched between first and second electrodes. An interval between the first and second electrodes continuously changes to continuously change the electric field strength between first and second electrodes, so that the phase transition between a crystalline state and an amorphous state can be controlled to the semiconductor to exhibit a desired resistance value.
Therefore, it is an object of the present invention to provide a current-limiting device that can be used as a resistor having a predetermined constant resistance at a normal state, but can limit the overcurrent even under an ambient temperature of about 500K in the event of an abnormality, i.e., the flow of an overcurrent.
Also, it is another object of the present invention to provide a current-limiting device which works satisfactory with no need to use any fuse and which is easily cooled so as to suppress any possible temperature-dependent change in resistance characteristic during a normal operating condition.
In accordance with one aspect of the present invention, there is provided a current-limiting device including a silicon substrate and two electrodes. The silicon substrate in turn includes an N type layer and two N+ layers that are formed respectively on opposite surfaces of the N type layer. The two electrodes are deposited on the opposite surfaces of the silicon substrate. The constant current flows in the current-limiting device if the applied voltage is higher than the predetermined value.
In other aspect of the present invention, the silicon substrate may include an Nxe2x88x92 layer and two N+ layers. The two N+ layers are formed on the opposite surfaces of the Nxe2x88x92 layer.
In further aspect of the present invention, the silicon substrate may include an Nxe2x88x92 layer, two N+ layers, and a plurality of P layers. The two N+ layers are formed on the opposite surfaces of the Nxe2x88x92 layer. The P layers are partially formed in each of the N+ layers.
The silicon substrate may include an Nxe2x88x92 layer, two N+ layers, and P layer. The two N+ layers are formed on the opposite surfaces of the Nxe2x88x92 layer. The P layers are formed between the N+ layer and the electrode.
The P layers may be subjected to a local life time control. The current-limiting effect may be functioned between 0OK and 500K. The two electrodes may be deposited on the opposite surfaces of the silicon substrate. The current-limiting device may further include two metallic electrodes by which the silicon substrate is sandwiched under pressure.
In another aspect of the present invention, the current-limiting device may further include the two metallic electrodes by which the silicon substrate is sandwiched under pressure.
According to the current-limiting device of this invention, there are resistance and current-limiting areas in the graph of current to applied voltage in the current-limiting device. In the resistance area, the current is nearly proportional to voltage and constant resistance is indicated if applied voltage is equal to or lower than the predetermined value. In the current-limiting area, the current is almost constant for current-limiting effect if the applied voltage is higher than the predetermined value for protecting the circuit from the overcurrent.
According to the current-limiting device of this invention, it may be made at a low price, such as the Nxe2x88x92 layer, as well as the above effect.
According to the current-limiting device of this invention, it may exhibit stable use of minimum heat dependence in the current-limiting area due to the P layers providing in it, as well as the above effects.
According to the current-limiting device of this invention, it may provide other embodiment and exhibit stable use of minimum heat dependence in the current-limiting area due to the P layers providing in it, as well as the above effects.
According to the current-limiting device of this invention, it may be controlled low resistance in the resistance area (normal operating state) because of local life time control, as well as the above effects.
According to the current-limiting device of this invention, it may protect the circuit from the overcurrent on 500K and under, as well as the above effects.
According to the current-limiting device of this invention, it may have low contact resistance between because of the electrodes directly providing on the silicon substrate, as well as the above effects.
According to the current-limiting device of this invention, it may have good electric contact and good heat conductivity, and it can exhibit a stable resistance property because of the metal electrodes, as well as the above effects.
According to the current-limiting device of this invention, it is pressed to the metal electrodes and a large area of the metal electrode is provided for cooling effect, so that a change of the resistance property due to heating can be prevented, and it can exhibit a stable resistance property. The current-limiting device can have a large cooling effect because it is pressed to the metal electrodes without glue, which may prevent heat conducting.